[keyserver] Use old intermediate invite link page url
Summary:
Currently, clients don't know how to handle new intermediate page urls (with comm:// scheme). After a new iOS release they will learn how to do it, so until then it is better to keep serving the old url.
Test Plan: Click a link and check if the app is opened.
Reviewers: kamil, inka
Reviewed By: kamil
Subscribers: ashoat
Differential Revision: https://phab.comm.dev/D8691